1. Field of the Invention
The present invention relates generally to electrical circuits and, more particularly, to charge pumps.
2. Related Art
Charge pumps are well known in the art. For example, a charge pump is commonly used in a frequency synthesizer, such as a phase-locked loop circuit. One drawback of a typical frequency synthesizer is charge pump leakage current, which contributes to frequency spurs and phase offset at the voltage-controlled oscillator output. For example, current mismatch in the charge pump current sources, due to the finite output impedance of transistors, results in differing current values when subjected to different drain-to-source voltage (e.g., complementary metal-oxide semiconductor technology) or collector-to-emitter voltage (e.g., bipolar junction transistor technology).
There are also other sources of leakage current at the loop filter terminal of a frequency synthesizer. As an example, a common-mode voltage setting circuit or the actual leakage of the filter components contribute to the leakage current.
The leakage current causes the discharge of the loop filter during the off-periods of the charge pump, resulting in a static phase offset between the input signals that drive the phase-frequency detector. The static phase offset is large enough to result in the injection of a charge each reference period, equivalent to the charge that escaped from the loop filter due to the leakage. The injection of charge into the loop filter each reference clock period creates frequency spurs at the voltage-controlled oscillator output.
Charge pump leakage current compensation systems and methods are disclosed herein. The charge pump leakage current compensation architecture reduces frequency spurs and phase offset in a frequency synthesizer by determining and compensating for the leakage current at the loop filter terminal.
In accordance with one embodiment of the present invention, a leakage current compensation circuit includes a frequency synthesizer and a timing circuit coupled to the frequency synthesizer. The timing circuit receives a clock signal from the frequency synthesizer and provides a timing signal indicating the timing of the clock signal relative to a reference clock signal. A counter, coupled to the timing circuit, provides a counter output value based on a value of the timing signal. A digital-to-analog converter, coupled to the counter and to the frequency synthesizer, receives the counter output value and provides a leakage current compensation signal to the frequency synthesizer based on the counter output value.
In accordance with another embodiment of the present invention, a method of compensating for leakage current in a frequency synthesizer includes monitoring a frequency synthesizer to determine when the frequency synthesizer is locked; determining a phase offset of the frequency synthesizer relative to a reference clock when the frequency synthesizer is locked; and supplying a leakage current compensation signal to the frequency synthesizer based on the phase offset determination.
A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.